Solar cells convert optical energy from sunlight into electricity. However, in order to be a viable energy source, the cost of the generated electricity must be competitive with alternative energy sources (e.g., coal, wind, nuclear, natural gas, etc.). One way of reducing net cost is to improve the efficiency of the solar cell. Efficiency is generally the percentage of sunlight (optical energy incident on the solar cell surface) that is converted into electrical energy.
There are various reasons why improving solar cell efficiency may be problematic. For example, because of the optical properties of materials used in the solar cell, such as silicon, a portion of the light incident on the solar cell surface may be reflected away and thus unavailable for the production of electricity. One method of reducing the amount of reflected light is the use of an anti-reflection coating (ARC). An ARC is a thin layer of a material (e.g. SiO2, TiO2 etc) that is grown on the surface of the solar cell (generally a silicon substrate or wafer). Optimizing the thickness and refractive index of the ARC reduces the amount of light reflected from the surface of the cell. Typically, an ARC coating is designed such that the reflection of light of a selected wavelength at normal incidence is reduced to zero. However, because incident light is comprised of various wavelengths of light, a substantial portion of incident light may still be reflected away.
An alternate technique involves texturing the surface of the wafer. Texturing generally involves etching the silicon wafer in a basic solution to form random upright peaks (e.g., pyramids, etc.) separated by valleys. Each peak generally includes an apex surface area and each valley includes a valley surface area. Consequently, larger peaks and/or deeper valleys have larger corresponding apex surface areas and valley surface areas.
Surface peaks and valleys tend to be very effective for trapping the light and minimizing front surface reflection. Typically, light that is reflected from an angled feature on a first pyramidal surface may be absorbed by second pyramidal surface, and thus used for the generation of electricity. In a typical solar cell a textured surface is combined with an AR coating to reduce the percentage of reflected light to around 1% for the wavelengths of interest.
Referring now to FIG. 1, a simplified diagram is shown comparing reflectivity values for a given wavelength of light incident on both a textured solar cell wafer and a non-textured solar cell with an ARC. Plot 102 shows the plot for the non-textured surface 102, while plot 102 shows a similar plot for a textured surface 104. In general, the ARC is a thin layer that uses destructive interference to reduce the reflection if incident light from the solar cell surface. Typically, an ARC layer thickness is chosen to be about one-quarter of a specific wavelength of light deep (λ/4). In this case, the ARC is optimized for 600 nm.
Consequently, if a first 600 nm beam is reflected off the top of the ARC, a second 600 nm beam will be reflected off the back of the ARC, traveling about half of its own wavelength (300 nm) further than the first beam. If the intensities of the two beams are exactly equal, then since they are exactly out of phase, they will destructively interfere and cancel each other. Consequently, there is no reflection from the surface, and all the energy of the beam must be in the transmitted into the solar cell. As can be seen, for a given wavelength of light where there is reflection, texturing substantially reduces the percentage of light reflected.
Unfortunately, the use of textured substrates presents significant challenges for the deposition of processing inks, such as for metallization and doping. That is, when a droplet of fluid is deposited on a surface, the drop is generally free to deform in shape and flow on the substrate surface. Its equilibrium shape is controlled not by the initial shape of the droplet, but by many additional factors such as the surface energy of the fluid, the contact angle the fluid makes with the substrate, gravity, the drying rate of the fluid and the morphology of the substrate surface. Comprehensive reviews of fluid interactions with surfaces can be found in R. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem, 28(6), 988-994 (1936) and P. G. de Gennes, Wetting: statics and dynamics, Rev. Mod. Phys 57(3), 827-863 (1985).
When the surface is a textured surface, the fluid behaves differently than it would on a flat surface. Because of surface roughness, the contact angle of the fluid on the textured surface is generally lower, consequently increasing the radius of the droplet. In addition, the fluid is generally free to flow away from the peaks into the valleys of the surface. Thus, the flow of fluid on textured surfaces is generally problem for forming conformal coatings and for generating patterns of fluid on the substrate.
Consequently, it would be of advantage to selectively modify the surface texture of a solar cell in order to enable the conformal coating and pattern retention of fluids on textured surfaces.